llvm dev - Nov 2013 - [LLVMdev] How to reduce the footprint of MDNodes? (About the comment you made at BOF LTO)

On Nov 12, 2013, at 1:28 PM, Chandler Carruth <chandlerc at google.com>
wrote:
> On Mon, Nov 11, 2013 at 11:29 PM, Chris Lattner <clattner at
apple.com> wrote:
> Hi Manman (and llvmdev),
> 
> I filed these two bugs to track the ideas that I was cooking:
> 
> http://llvm.org/bugs/show_bug.cgi?id=17891
> http://llvm.org/bugs/show_bug.cgi?id=17892
> 
> TL;DR: I'm saying we should go from:
> 
>         !14 = metadata !{i32 786445, metadata !1, metadata !10, metadata
!"y", i32 3, i64 32, i64 32, i64 32, i32 0, metadata !13}
> to:
>         !14 = metadata !"v12,14,y,3,0,32,32,32"(metadata !1,
metadata !13)
> 
> So, I like where you're going here, but a few tweaks.
> 
> First, there are two things going on here: removing an indirection through
a referenced metadata node and flattening N values into a string inclusion.
Removing the indirection seems obvious strict goodness, my comments are about
the second part.
David pointed out that the PR17892 is a microoptimization that may not even be
worthwhile.  I think that the flattening into a string is the important part.
> I'm moderately opposed to just encoding these in a string format. I
think we can do something substantially better both for space, time, and
readability. Fundamentally, there is no reason for the original metadata node
you describe to not *encode* its operands into a dense bit-packed blob of
memory. We can still expose APIs that manipulate them as separate entities, and
have the AsmPrinter and AsmParser map back and forth with nice human-readable
forms. But even a simple varint encoding will be both smaller and faster than
ascii.
I guess you could make it work, but would that actually be simpler than what is
proposed?  If it is denser, how much denser would it have to be to justify the
complexity?
> Just to be clear, I still want the nice format (much like your proposed
format, but maybe with the numbers outside of the "s) in the textual IR, I
just think we should use a more direct and efficient in-memory encoding (and
in-bitcode encoding if that isn't already suitably dense).
Where would the encoding schema be specified?

Note that there are simple things that can be done to make MDNodes more
efficient in common cases.  The CallbackVH is only necessary when pointing to
Value*’s that are not MDNode/MDString, and Constants-other-than-GlobalValue.  If
we make MDNode detect when it has “all-immortal” operands (like most debug info
nodes) then we could just store Value*’s directly.  This would be a completely
invisible implementation improvement, but would not provide the same level of
improvement as the “flatten into strings” approach.  The two are quite
complementary.

-Chris
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On Tue, Nov 12, 2013 at 4:14 PM, Chris Lattner <clattner at apple.com>
wrote:
> I'm moderately opposed to just encoding these in a string format. I
think
> we can do something substantially better both for space, time, and
> readability. Fundamentally, there is no reason for the original metadata
> node you describe to not *encode* its operands into a dense bit-packed blob
> of memory. We can still expose APIs that manipulate them as separate
> entities, and have the AsmPrinter and AsmParser map back and forth with
> nice human-readable forms. But even a simple varint encoding will be both
> smaller and faster than ascii.
>
>
> I guess you could make it work, but would that actually be simpler than
> what is proposed?  If it is denser, how much denser would it have to be to
> justify the complexity?
>
I don't think it would be more complex than a string encoding. At least,
I'm not imagining we want to be super clever here.

I could even imagine doing a versioned giant bitfield and using the version
to handle auto-upgrade...

>
> Just to be clear, I still want the nice format (much like your proposed
> format, but maybe with the numbers outside of the "s) in the textual
IR, I
> just think we should use a more direct and efficient in-memory encoding
> (and in-bitcode encoding if that isn't already suitably dense).
>
>
> Where would the encoding schema be specified?
>
Same question applies to a string encoding. We have to define the schema
somewhere clearly. I'm just lobbying for the textual IR and the APIs to
both operate directly on N fields, and just make the memory representation
dense.

>
> Note that there are simple things that can be done to make MDNodes more
> efficient in common cases.  The CallbackVH is only necessary when pointing
> to Value*’s that are not MDNode/MDString, and
> Constants-other-than-GlobalValue.  If we make MDNode detect when it has
> “all-immortal” operands (like most debug info nodes) then we could just
> store Value*’s directly.  This would be a completely invisible
> implementation improvement, but would not provide the same level of
> improvement as the “flatten into strings” approach.  The two are quite
> complementary.
>
Yea, I'd rather go for at least a bit more dense than that, but maybe we
should do this step-by-step.
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On Tue, Nov 12, 2013 at 4:19 PM, Chandler Carruth <chandlerc at
google.com>wrote:
>
> On Tue, Nov 12, 2013 at 4:14 PM, Chris Lattner <clattner at
apple.com> wrote:
>
>> I'm moderately opposed to just encoding these in a string format. I
think
>> we can do something substantially better both for space, time, and
>> readability. Fundamentally, there is no reason for the original
metadata
>> node you describe to not *encode* its operands into a dense bit-packed
blob
>> of memory. We can still expose APIs that manipulate them as separate
>> entities, and have the AsmPrinter and AsmParser map back and forth with
>> nice human-readable forms. But even a simple varint encoding will be
both
>> smaller and faster than ascii.
>>
>>
>> I guess you could make it work, but would that actually be simpler than
>> what is proposed?  If it is denser, how much denser would it have to be
to
>> justify the complexity?
>>
>
> I don't think it would be more complex than a string encoding. At
least,
> I'm not imagining we want to be super clever here.
>
> I could even imagine doing a versioned giant bitfield and using the
> version to handle auto-upgrade...
>
>
>>
>> Just to be clear, I still want the nice format (much like your proposed
>> format, but maybe with the numbers outside of the "s) in the
textual IR, I
>> just think we should use a more direct and efficient in-memory encoding
>> (and in-bitcode encoding if that isn't already suitably dense).
>>
>>
>> Where would the encoding schema be specified?
>>
>
> Same question applies to a string encoding. We have to define the schema
> somewhere clearly. I'm just lobbying for the textual IR and the APIs to
> both operate directly on N fields, and just make the memory representation
> dense.
>
The difference here is that debug info parsing code would know the schema
externally - so the metadata itself wouldn't have to be self-describing or
typed in any way. Just a flat series of bytes of a fixed size would be
sufficient. (then leaving out the fields that refer to other IR constructs
such as functions, variables, etc)

But if we could make general metadata generally more compact that'd be nice
too and maybe sufficient/instead/not worth the added complexity in debug
info code of pulling out fields in the debug info handling code.

>
>
>>
>> Note that there are simple things that can be done to make MDNodes more
>> efficient in common cases.  The CallbackVH is only necessary when
pointing
>> to Value*’s that are not MDNode/MDString, and
>> Constants-other-than-GlobalValue.  If we make MDNode detect when it has
>> “all-immortal” operands (like most debug info nodes) then we could just
>> store Value*’s directly.  This would be a completely invisible
>> implementation improvement, but would not provide the same level of
>> improvement as the “flatten into strings” approach.  The two are quite
>> complementary.
>>
>
> Yea, I'd rather go for at least a bit more dense than that, but maybe
we
> should do this step-by-step.
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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On Nov 12, 2013, at 4:19 PM, Chandler Carruth <chandlerc at google.com>
wrote:
> On Tue, Nov 12, 2013 at 4:14 PM, Chris Lattner <clattner at
apple.com> wrote:
>> I'm moderately opposed to just encoding these in a string format. I
think we can do something substantially better both for space, time, and
readability. Fundamentally, there is no reason for the original metadata node
you describe to not *encode* its operands into a dense bit-packed blob of
memory. We can still expose APIs that manipulate them as separate entities, and
have the AsmPrinter and AsmParser map back and forth with nice human-readable
forms. But even a simple varint encoding will be both smaller and faster than
ascii.
> 
> I guess you could make it work, but would that actually be simpler than
what is proposed?  If it is denser, how much denser would it have to be to
justify the complexity?
> 
> I don't think it would be more complex than a string encoding. At
least, I'm not imagining we want to be super clever here.
> 
> I could even imagine doing a versioned giant bitfield and using the version
to handle auto-upgrade…
You must mean something other than I’m imagining :-).  From your description, I
think you’re describing that we have some new kind of “compressed mdnode” that
bitpacks data in some way, and that we’d have the existing .ll and .bc syntax be
magically compacted without a syntax change.  Is that what you’re describing?

If so, that sounds really complicated: all of the readers would have to
recognize the new format, and we lose alignment of in memory IR with the printed
form (making debugging the compiler simpler).

If you’re talking about exposing this as syntax in .ll files (and encoding
details in .bc files) then I’m not sure how this would work.  You’d have to have
the schema for each node described somewhere.  Where would this exist.

More generally, can you explain more of what you’re thinking here?
>  
>> Just to be clear, I still want the nice format (much like your proposed
format, but maybe with the numbers outside of the "s) in the textual IR, I
just think we should use a more direct and efficient in-memory encoding (and
in-bitcode encoding if that isn't already suitably dense).
> 
> Where would the encoding schema be specified?
> 
> Same question applies to a string encoding. We have to define the schema
somewhere clearly. I'm just lobbying for the textual IR and the APIs to both
operate directly on N fields, and just make the memory representation dense.
The advantage of strings is that it moves the schema complexity to the debug
info - related machinery like DIBuilder.  The core IR features like MDNode, the
asmparser/writer, bitcode support, etc don’t need anything specific to debug
information.

-Chris
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